1. When did I last use this item?
If you haven't used it in the last year, chances are your not going to in the next. Allow yourself to purge old items, interest, hobbies, and unfinished projects to allow room for the really important parts of your life. Try letting it go to someone who really wants, or could use the item. If you are protesting too much that you might someday need the item, move on to question #2. This will help you deide if it should continue to be stored in your home.
2. Am I going to need this item on a specific date in the future?
If your best friend just set her wedding date for a couple of months away, it might be wise to keep your mint in the box punch bowl for future use. But if you are keeping a household full of furniture stacked in the garage for when your 5 year old daughter moves out on her own, it may be time to free up some space. If it doesn't have a definite date of use, the clutter isn't worth it. Be diligent about this. I know that sometimes we feel like we are saving money later by keeping items now. Realize that the space and maintenance of those items makes them expensive now to keep.
3. Does this item have a legal or tax purpose?
Paper can be our worst clutter enemy and we often use the excuse of the legal and tax purposes associated with a thirty year collection of receipts. Go through your papers and keep only items that are really needed. Even better, set up a systems now to deal with paper on a daily basis. This will keep thsoe paper piles from forming in the first place.
4. How hard would it be to replace this item again someday if I needed it?
If items are priceless emotionally or historically, they may not be the best items to toss. Consider how hard it would be to replace the item and get rid of things that are a dime a dozen. Remember not to fall into the trap of #3, keeping items for a future use that is too far into the future.
5. In what way is this item making my life more purposeful or better?
If the items are not making you happier, healthier, or better, they are just taking up space and cluttering up your life. If the items only create more things to clean and organize your home, it's time to let them go. Removing these stacks and boxes of stuff will free up your home and your space.